Refections on transparent object - renderd view diffrent to render
This is an old bug in blender. You are doing nothing wrong. The information is there, but the render window is not displaying it correctly. It cannot display pixels that are transparent and luminous at the same time. Glares, glows, reflections. and fire don't display correctly.
View your image just as color (not color and alpha)

Your information should be there. To save it use a format like OpenEXR
For more info read:
How do I get the Glare node to output transparent instead of Black background